The Epidermal Growth Factor (EGF) receptor or ErbB family of receptor tyrosine kinases (RTKs) have been implicated in many human cancers. It is well established that antibodies to the extracellular region of the EGF receptor (EGFR) that prevent ligand binding and/or receptor signaling can inhibit tumor growth in vivo. At least 5 such anti-EGFR antibodies are in clinical trials; 1, cetuximab/Erbitux, was FDA-approved to treat advanced colon cancer in February 2004. Recent structural studies of EGFR family extracellular regions have identified a novel mechanism for ligand induced receptor dimerization. The unliganded receptor is found in a characteristic "autoinhibited" configuration in which the receptor dimerization interface is completely occluded in an intramolecular "tether". Ligand binding induces a dramatic conformational change in EGFR that exposes this normally buried extracellular dimerization site, promoting dimerization and receptor activation. We will analyze how particular antibodies against the EGFR extracellular region (sEGFR) can have dramatically different effects. Certain antibodies inhibit EGFR activation, while others alter (or bias) the affinity state of the receptor at the cell surface, sometimes allowing signaling to proceed. Still others specifically impair binding of 1 ligand without affecting binding of another. We hypothesize that these different effects reflect the distinct extracellular conformations stabilized by the particular antibodies. We will analyze antibody binding to sEGFR mutants with defined altered conformations, and use X-ray crystallography to determine structures of Fab:sEGFR complexes. Finally, in an effort to exploit recent structural advances, we will screen phage display libraries for single-chain Fv (scFv) molecules that bind to key epitopes in sEGFR and that we hypothesize will inhibit EGFR in novel ways. We hope to gain structural insight into relationships between extracellular conformation and activity, affinity and specificity of EGFR at the cell surface. The specific aims are: 1. To investigate relationships between EGFR responsiveness and extracellular domain conformation by analyzing the structural consequences of binding antibodies with known effects on in vivo receptor function. 2. To probe ligand specificity in EGFR activation using an antibody that selectively inhibits binding of a subset of EGFR agonists.